3 Answers
3

Well, even if you don't like it, I will put you to read again with more attention man time. At the end of EXAMPLES section you will find:

Users of the bash shell need to use an explicit path in order to run
the external time command and not the shell builtin variant. On system
where time is installed in /usr/bin, the first example would become
/usr/bin/time wc /etc/hosts

So, I'm assuming that you use bash shell which uses an internal version of time, provided as a shell keyword. You can check this using the following command:

type time

and the output will probably be:

time is a shell keyword

If this is the case, then is clear, to use the realtime command, you must to use its explicit path: /usr/bin/time.

Since, as the other answers explain, time is a shell keyword, the only option available to you is -p:

terdon@oregano ~ $ help time
time: time [-p] pipeline
Report time consumed by pipeline's execution.
Execute PIPELINE and print a summary of the real time, user CPU time,
and system CPU time spent executing PIPELINE when it terminates.
Options:
-p print the timing summary in the portable Posix format

So, you need to run the time that's in /usr/bin. Here are a few ways to do so:

Use the time executable instead:

/usr/bin/time -f %Uuser ls >/dev/null

Use \ which causes your shell to ignore aliases and keywords and instead search your $PATH for a matching executable:

\time -f %Uuser ls >/dev/null

Use the command builtin which has a similar effect:

command time -f %Uuser ls >/dev/null

Use a different shell, one that has no such keyword. For example sh (which is actually dash on Ubuntu):

sh -c "time -f %Uuser ls >/dev/null"

Use which, which will search through your $PATH (OK, this one is silly):

Is there a way to do the opposite - replicate built-in time with regular /usr/bin/time? I.e. to translate bash script into dash.
– Dan M.Sep 26 '19 at 12:06

@DanM. I don't understand. If you want the builtin time, run time. If you want the standalone executable, run /usr/bin/time.
– terdonSep 26 '19 at 12:10

there is no built-in time on Ubuntu's default shell as far as I'm aware. I want to translate script using built-in into standalone with the same behavior.
– Dan M.Sep 26 '19 at 12:13

@DanM. then just use time. It will call whichever one is available. That will be the builtin for bash and the standalone for dash. If you use /usr/bin/time, then it will always call the standalone, both in bash and in dash. But it sounds like you should probably be asking a new question, explaining the behavior you need. Let me know if you do and I'll try and help.
– terdonSep 26 '19 at 12:18

yeah, but built-in and standalone have drastically different syntax/semantics. Anyway, thanks. I'll ask a new question if I won't figure out how to deal with it myself.
– Dan M.Sep 26 '19 at 12:31